1. Field of the Invention
The invention relates to a sound pickup method and system, more particularly to a sound pickup method and system that employs sound source tracking to enhance sound pickup quality of a microphone array.
2. Description of the Related Art
A conventional microphone array includes a plurality of microphones disposed in an array and spaced apart from each other. By processing sound source signals picked up by the microphones, directionality of the sound source signals can be determined. As such, the microphone array can be used to promote signal-to-noise ratio (abbreviated as SNR) so as to enhance a target signal that originates from a specific direction by suppressing noise from other directions.
Referring to FIG. 1, a conventional so-called delay-and-sum microphone array 1 is shown to include a number (n) of microphones 11 disposed in an array, a number (n) of delay units 12, each of which is coupled to a corresponding microphone 11, and an adder 13 connected to the delay units 12. Adjacent ones of the microphones 11 are spaced apart by a distance (d). When each of the microphones 11 receives a sound source signal, the corresponding delay unit 12 will perform corresponding signal delay for the sound source signal in accordance with predetermined estimated delay times, such as Δt1, Δt2 and Δt3, in sequence. For example, the signal received by the first microphone (m1) will be transmitted to the adder 13 after a delay time (n−1)×Δt1, the signal received by the second microphone (m2) will be transmitted to the adder 13 after a delay time (n−2)×Δt1, and so on. The delayed signals will be subsequently combined in the adder 13. Hence, for the predetermined estimated delay times Δt1, Δt2, and Δt3, the combined signal can be expressed as one of:
                                                                                          y1                  ⁡                                      (                    t                    )                                                  =                                                      ∑                                          k                      =                      1                                        n                                    ⁢                                                            x                      k                                        ⁡                                          (                                              t                        +                                                                              (                                                          k                              -                              1                                                        )                                                    ×                          Δ                          ⁢                                                                                                          ⁢                          t1                                                                    )                                                                                  ,                                                                                                            y2                  ⁡                                      (                    t                    )                                                  =                                                      ∑                                          k                      =                      1                                        n                                    ⁢                                                            x                      k                                        ⁡                                          (                                              t                        +                                                                              (                                                          k                              -                              1                                                        )                                                    ×                          Δ                          ⁢                                                                                                          ⁢                          t2                                                                    )                                                                                  ,              and                                                                                    y3            ⁡                          (              t              )                                =                                    ∑                              k                =                1                            n                        ⁢                                          x                k                            ⁡                              (                                  t                  +                                                            (                                              k                        -                        1                                            )                                        ×                    Δ                    ⁢                                                                                  ⁢                    t3                                                  )                                                    ,            
Then, from the combined signals y1 (t), y2 (t) and y3 (t), a signal having the largest amplitude is determined so as to obtain an indication of the loudest sound source. As such, a delay time At defined as the time difference between the time when the signal of the loudest sound source reaches a microphone nearest thereto and the time when the signal of the loudest sound source reaches another microphone adjacent to the nearest microphone is obtained. By the formula: d×sin θ=v×Δt, where v is the velocity of sound, the direction and the angle θ of the loudest sound source can be calculated. After the delay time Δt is obtained, the delay units 12 are operated to delay the sound source signals of the corresponding microphones 11 in accordance with the delay time Δt. In this manner, signals from the loudest sound source are enhanced while suppressing signals from sound sources in other directions.
From the foregoing, it is apparent that the conventional microphone array 1 is able to find the direction of a loudest sound source and to enhance signals picked up from the loudest sound source. However, in situations where the noise amplitude is greater than that of a target sound source signal (i.e., the loudest sound source is not the target sound source), the undesired noise signal will be enhanced while suppressing the target sound source signal, thereby resulting in poor sound pickup quality.